Glossmeter



June 1941. D. B. BRADNER EI'AL 2,245,501

GLOSSMETER Filed March 1, 1939 "/0 INVENTORS ATTO RNEIYS Patented June24, 1941 2,246,501 oLossMn'rna Donald B. Bradner and George C. Munro,Hamilton, Ohio, assignors to The Champion Paper and Fibre Company,Hamilton, Ohio, a corporation 01 Ohio OFFICE Application March 1, ram,serial No. stairs tlClaims. (or rarll This invention relates toglossmeters, and more particularly concerns improvements in methods andapparatus for measuring gloss which make possible the measurement ofgloss on a moving web of paper. regardless of its speed of movement.

One of the most important properties of paper is its gloss, and moreparticularly the uniformity of the degree of gloss it possesses. Thegloss characteristic is indicative to a great degree, not only of theappearance and feel of the paper, but also of the surface smoothness,density, and other or the properties which determine its printingquality. Thus if the manufacture of paper can be controlled to give it auniform degree of gloss throughout, such control results in an enhanceduniformity of these other properties and consequently greatly increasesits value to the printer.

lit has heretofore been the practice to take samples periodically fromthe paper being produced, and compare their gloss with that of a sampleto be matched, but in spite of the utmost care the gloss frequentlyvaries considerably between samples taken, which therefore do not give ar liable index of the gloss throughout the length of the web. Attemptsto decrease the space between samples taken give little better resultsand further seriously cut down production.

The prime requisite of an improvement in the uniformity of glossthroughout the length of a paper web is therefore a method and apparatuswhich will make it possible to get a substantially continuous, or atleast a very frequent measurement of gloss on the web being producedwithout stopping production, breaking the web, or retarding its speed ofmovement.

The continuous measurement of the gloss on a paper web is a difficultproblem. At usual paper web speeds, which may be of the order of 1,000feet per minute, the web issuing from the calenders vibrates rapidly andthe necessary web tension tends to produce longitudinal wrinklestherein. This is particularly true of light or moderate weight printingpapers. Further, it is diihcult to obtain gloss measurements over anarea of the web which is large enough to be indicative of the glosscharacteristic over the entire web surface, since the paper glosscharacteristic usually varies considerably from foot to foot and evenfrom inch to inch of the paper surface.

With the above considerations in mind, it is the primary object of thepresent invention to provide a method and means for continuously andaccurately detecting and measuring the gloss characteristic of a rapidlymoving paper web, such as a web of paper issuing from a calender, themethod and means being of such a nature as to respond to the glosscharacteristic of the paper over areas large enough to be representativeof the entire paper web surface. An additional object is to provide amethod and means of gloss measurement which corresponds closely with thevisual gloss ratings made by those skilled in the paper finishing art,regardless of the color of the web. Other detailed objects of theinvention will be apparent from the tollowing description.

The method of gloss measurement which we prefer to use in connectionwith the present invention comprises directing a light beam of known orcontrolled intensity at an angle onto the surface whose gloss is to bemeasured and measuring the intensity of the beam after specularreflection from the surface. For measuring gloss on many grades ofpaper, high angles of incidence are of advantage-for example, angles ofmore than degrees, and advantageously about degrees with the normal tothe surface, have been found to give results most closely approachingskilled in the art, and least afiected by the color of the surface. Themeasurement is ordinarily made photoelectrically, and. to obviatefluctuations in the readings due to the variations in gloss on differentparts of the moving web, we damp the variations in the current in thephotoelectric circuit. In some cases, the gloss may be measured bydetermining the ratio of the specular to diffuse reflected light, butregardless of the particular method employed the output of the photocellshould be damped.

The apparatus of a preferred embodiment of the invention includes meansfor supporting the paper web in a fixed and advantageously convex form,as by passing the moving web over a cylindrically convex surface such asa truly running roll. We prefer to use a cylindrical optical element inthe path of the gloss measuring light beam to compensate for thedispersive efiect of the convex form imparted to the paper web surfaceby its supporting means. The portion of the light beam specularlyreflected by the paper surface is received by suitable light sensitivemeans such as a photoelectric cell whose output furnishes the desiredindication of the gloss characteristic of the paper. The form andarrangement of the elements comprising our improved glossmeter, ashereinafter described, provide a gloss measurement over a sumcient widthof the visual ratings by those paper web to be fairly representativeofthe gloss of the entire web surface.

One preferred embodiment" of the invention will be described byreference to the accompanying drawing in which;

Figure 1 is a diagrammatic view showing the general arrangement of theapparatus in relation to the moving paper web;

Figure 2 is a diagrammatic .end view showing how the light rays arereflected by the cylindrical mirror and the cylindrical paper surface;

Figure 3 is a fragmentary view showing the changes in the structurerequired when using a cylindrical lens instead of the cylindricalmirror; and

Figure 4 is a fragmentary view showing the l cation of the photoelectriccell when using a cylindricaLmirror in the path of the reflected ratherthan of the incident light beam.

In the disclosed embodiment of the invention, the surface of the paperweb in issuing from a calender or other paper making or finishingmachine, is held in a fixed form at any convenient location, by passingit over a freely turning, accurate, and truly running fly roll I I, withthe side on which the gloss is to be measured away from the roll. Thisgives a cylindrically convex form to the surface on which themeasurement is to be made. The tension on the web tends to hold it inintimate contact with-the surface of the roll and thereby to give adefinite geometrical form of known optical properties to the papersurface at this location. The angle of wrap of the paper around the rollshould be sufficient to ensure that the entire illuminated area of paperwill be given the desired cylindrical form.

Light is provided from any suitable source, here shown as anincandescent lamp i2 whose filament provides a high intensity source ofsmall area. The lamp shown is operated on'a low voltage circuit on whichthe voltage is kept constant by a constant voltage transformer or otherknown means, not shown. An adjustable resistance I3 is placed in thelamp circuit for adjusting the intensity of the light.

In order to make certain that the intensity of the light remains at aconstant predetermined value, a photoelectric cell I4 is placed where itis continuously illuminated by the lamp H. The cell i4 is connected to ameter i5 which may be adapted to continuously indicate, or record, orboth, the intensity of the light. Variation from standard intensity dueto aging of the lamp or other cause is thus immediately indicated andcan be corrected by adjusting resistance i3.

Light from the source I2 is collimated by a condenser lens 20, or othersuitable means. The substantially parallel beam 21 from the condenserfalls upon the cylindrically concave surface of the mirror 22. The lightbeam 23 reflected. from the mirror remains substantially parallel in thevertical projection illustrated in Figure 1, but converges, in theprojection illustrated in Figure 2, towards the focal line 24 of themirror 22. The mirror 22 is located so that its focal line 2 53coincides with the virtual focal line of the cylindrical surface of theweb ill on roll ii. Consequently that part of the light in incident beam23 which is specularly reflected from the surface of the web l0 on rollll, leaves the cylindrical surface of the paper as a substantiallyparallel beam 26.

In the path of the beam 26, is placed a photoelectric cell 30 which isprotected from extraneous light by suitable means such as a cylindrical(Figure 2) shield 3!. An electrically responsive meter 32 of anysuitable type may be used to measure and visually indicate the output ofcell ,30 and thereby the gloss on the web, or a recorder may be added tothe meter 32 to give an autographic, advantageously a continuous recordof the gloss on the moving web. A resistance 33 is placed across theterminals of the meter32. The value of this resistance is chosen withreference to the resistance of the meter 32 to provide means forelectrically damping fluctuations in the current from photocell 30. Inplace of this electric damping element suitable mechanical damping meansmay be provided in .the meter 32 in a manner well known in the art.

As can be seen from Figure 1, the incident light beam 23 is arranged tocover a considerable area along the length of the roll ll, 1. e., acrossthe width of the moving webi' The width which needs be covered is notfixed, but is preferably over three inches and is a dvantageously notless than nine inches in order that the gloss reading may showtheaverage over a fairly representative area of the paper surface. Thus ona thirty-six inch wide web the gloss reading may represent the actualaverage gloss of more than 25 per cent of the entire area of finishedpaper, and truly representative gloss measurements are accordinglyobtained.

It should be noted that, as shown in Figure 2, the mirror 22concentrates the light beam to cover a very small arc 25 on thecircularly cylindrical surface imparted to the web ill by the roll i l.Because of the direction of paper travel, narrowing the beam in thisdirection does not decrease the paper area on which the gloss ismeasured. This concentration of the beam not only increases theintensity of illumination, but, by virtue of the small arc, the surfaceso nearly approximates a parabola that it has an approximately truevirtual focus at the line 24 parallel to the roll axis, and thespecularly reflected beam 26 is much more nearly parallel than ifreflected from a larger circle arc.

It is possible in accordance with the invention to substitute anequivalent cylindrical lens 42 (Fig. 3) for the cylindrical mirror 22,by arranging the lens to direct the incident light beam 23 to the focalline 24 of the cylinder, as was done with the mirror 22. With the highangles of incidence found advantageous for measuring gloss however, Igenerally prefer to use the mirror because it involves a smaller lightloss.

It is also possible to introduce a cylindrical lens or mirror into thepath of the specularly reflected beam 25 instead of the incident beam2|, and still secure a measure of compensation. However, if the parallelbeam 21 is incident directly on the paper it on roll ll, specularreflection from the cylindrical surface disperses the beam as indicatedby the broken lines 21-21 In this case, the reflected rays no longerappear to issue from a virtual image at 24, but appear to come from anarea between 24 and 28. To gather a substantial portion of thisdispersed light, the cylindrical lens or mirror has to be much wider andcloser to the roll i0 than is mirror 22. The reflected beam, even ifrendered parallel thereby, is much broader and much less intense thanwith the arrangement illustrated in Figure 1. Furthermore, in order tosecure even approximate compensation, special forms of lenses or mirrorsare required, and it is further desirable to narrow the incident beam orgreatly increase the diameter of the roll H, or

both, to reduce the arc illuminated and cut down theangularity ofdispersion 21. In case a mirror 52 is used in the path of the reflectedbeam, it is, of course, necessary to relocate the photoelectric cell 30so that it receives the beam from the mirror, as shown in Figure 4.

Instead of passing the paper over roll II, it would also be possible todrag the web o 'er 9, cylindrically convex surface of parabolic or anyother desired form. It is simply necessary to make the cylindrical lensor mirror of proper form to at least approximately compensate for thedispersive effect of the convex surface on which the measurement is tobe made. While cylindrical lenses can conveniently be made to compensatefor only a limited number of forms over 9 inches along the length ofroll II. Measof convex surface, itis possible to design a cylindricalsurface. This arrangement permits measurement of the gloss in thedirection of the grain, as it is commonly judged visually. In such casethe angle of incidence should be held within the limits specified andthelight should be directed onto the paper in such a manner that the lightspecularly reflected from the paper will be in the form of asubstantially parallel beam. In the specific embodiments described, thespecularly reflected light has been in the form of a substan:

tially parallel beam. Although thishas a number of advantages, thespecularly reflected light may be in the formof a divergent orconvergent beam so long as variations in the specularly reflected lightcorresponding to gloss changes are suillcient to actuate the photocell.i

I Although anyof the known types of photoelectric cell may be used, thedevice is simpler and the results are more accurate when using a type ofcell which does not require the application of potential from a sourceoutside the cell.

The type of cell described in United States Patent No. 2,034,334, forexample, has been found satisfactory.

To facilitate the understanding of a construc tion and arrangement ofparts which makes a1 gloss measurement over a wide area of the web,

the following details of a specific embodimen typifying the inventionare given.

The roll II is adapted totake a web up to 60 inches in width, is about5.9 inches in diameter and its surface hasa virtual focus approximate-1y 1.475 inches below its outer surface. Lens is a paraboloidal lens of.60 mm; focal length, and

ured along the axisoi the beam, thedistance from lens 20 to mirror 22 isappioximately 7% inches. that from mirror 22 to roll I I isapproximately 26% inches, and that from roll ll to photocell 30 isapproximately the same. measurements of gloss secured with thisarrangement have been found to be dependable and repeatable and to checkwell with the visual gradings made by those skilled in the ar As pointedout above, it is preferredto locate the photoelectric cell 30 at aconsiderable distance from the area of the paper on which the light isreflected, and we have found that this distance may advantageously be atleast ten times the greatest dimension of the exposed sur face of thephotoelectric cell. With these proportions, relatively little of thelight diffusely refiected from the paper surface reaches thephotoelectric cell, and the readings are found to check closely with thevisual judgments of experts in verging element, designates the linetowards "surface, designates the line from which parallel 2 5' inchesdiameter. The lamp" is a -candle I power incandescent. lampyof. a typesimilar to those used for automobile headlamps, and is lo catedapproximately at the focal point of lens 20 or very slightly beyond. Theconcave mirror 22 is approximately 9 inches long and 2% inches wide andhasa radius of curvature of approximately 16.692 inches and focal lengthof approximately 8.346 inches. It is set so that its light rays,directed backward along the path of the specularly reflected beam in theinstrument, will be made to appear to diverge by specular reflectionfrom the surface. As is well known,

aberrations which vary with the effective aperture, prevent these frombeing true geometrical lines, but it is to be understood that, as in.all.

optical instruments, the apertures or width of arc illuminated are solimited as to prevent these aberrations from exceeding practical limits.

In the appended claims the expression a moving paper we refers to atraveling web of paper continually presenting a new surface incontradistinction to a sheet of paper which may be pasted or otherwiseheld to a movingsupportandwhich recurrently presents the same sur-- faceto the gloss measuring device.

By describing our invention in connection with the measurement of glosson paper, we do not preclude the use thereof for the measurement ofgloss on other materials to which it may be focal line is substantiallycoincident. with the virtual focal-line of the surface of the paper 23makes an angle of approximately 15 degrees with the-axis of theroli IIand covers an area found applicable.

ly parallel rays and a cylindrically concave mir ror in the path of thecollimated beam, said mirror being set with its focal line approximatelyThe i the light from said beam which is specularly reflected by thepaper.

, 2. In a device for measuring gloss on paper, in combination with amoving paper web, a fixed cylindrical paper support for giving "acylindrical form to the paper where its gloss is to be measured, a lightsource, means for directing a beam of light from said source onto thepaper web on said cylindrical support in a plane substantially axial tosaid support, said light directing means including means for collimatinglight into a beam of substantially parallel rays and a cylindricallyconcave mirror in the path of the collimated beam, said mirror being setwith its focal line approximately coincident with the virtual focal lineof the cylinder formed by paper on said support, wherebisaid light beam,after reflection by said mirror,'converges towards said virtual focalline, and means for measuring the intensity of that part of the lightfrom said beam which is specularly reflected by the paper.

3. A device for measuring gloss on paper comprising a cylindricallyconvex support for giving a definite. form and position to the paperwhose gloss is being measured, a light source, a lens for collimatinglight from said source into a beam of substantially parallel rays, acylindrical light converging element disposed parallel to said supportand in the path of said beam with its focal line approximatelycoincident with the virtual focal line of the cylinder formed by thepaper on said support, whereby that part of the lightbeam which isspecularly reflected by the paper on said support is in the form of asubstantially parallel beam, and means for measuring the intensity ofthat part of the light from said beam which is specularly reflected bythe paper.

4. In a device for measuring gloss on a moving web, a truly runningcylindrical roll over which the web travels to give it a fixed form andtion to converge to a substantially rectilinear 10- cus which issubstantially coincident with the virtual focal line of the illuminatedarea of the convex surface of the paper, whereby that part of said beamwhich is specularly reflected by said surface leaves said surface as asubstantially parallel beam, and measuring the intensity of saidspecularly reflected beam.

6. The method of measuring gloss on a moving web, which comprisespassing the web in contact with a cylindrically convex surface to giveit definite form and position, collimating light from a small area lightsource to form a substantially parallel ray light beam, concentratingthe light rays in said beam in one direction to converge to asubstantially rectilinear focus which is substantially coincident withthe virtual focal line of the illuminated area of the cylindricallyconvex position where its gloss is to be measured, a light source, meansfor collimating light from said source into a beam of substantiallyparallel rays,

a cylindrical light converging element mounted parallel to said roll inthe path of said beam with its focal line approximately coincident withthe virtual focal line of the cylinder formed by the web on said roll,said cylindrical light converging element being so disposed that theconvergent light from said beam is caused to travel in a plane axial tosaid roll and to strike the cylindrical surface of the web on said rollat an angle of less than thirty degrees therewith, and to illuminate anarea thereon which is long in the direction of the roll axis and narrowin the other direction, whereby that part of the light which isspecularly reflected by the cylindrical surface of the web emergestherefrom as a substantially parallel beam, a photoelectric cellpositioned to receive the specularly reflected beam and means formeasuring the output of said cell.

5. The method of measuring gloss on paper, which comprises bending thepaper around and in contact with a cylindrically convex surface to giveit definite form and position, collimating light from a small area lightsource to form an approximately parallel ray light beam, concentratingthe light rays in said beam in one direcsurface of the web, directingsaid convergent light beam onto said cylindrical surface at an angle ofless than thirty degrees therewith and in a plane substantially axialthereto, whereby that part of said beam which is specularly reflectedfrom said convex surface emerges therefrom as an approximately parallelbeam in the same axial plane, and measuring the intensity of saidspecularly reflected beam.

'7. The method of measuring gloss on a moving web, which comprisespassing the web in contact with a cylindrically convex surface to giveit deflnite form and position, collimating light from a small area lightsource to form a substantially parallel ray light beam, concentratingthe light rays in said beam in one direction to converge to asubstantially rectilinear focus which is substantially coincident withthe virtual focal line of the illuminated area of the cylindricallyconvex surface of the web by projecting said beam onto a cylindricallyconcave mirror, directing said convergent light beam onto saidcylindrical surface at an angle of less than thirty degrees therewithand in a plane substantially axial thereto, whereby that part of saidbeam which is specularly reflected from said convex surface emergestherefrom as an approximately parallel beam in the same axial plane, andmeasuring the intensity of said specularly reflected beam.

8. The method of measuring gloss on a moving Web, which comprisespassing the web in contact with the convex surface of a right circularcylinder to give it definite form and position, collimating light from asmall area source to form a substantially parallel light beam,concentrating the light rays in said beam in one direction to convergeto a substantially rectilinear focus which is substantially coincidentwith the virtual focal line of the illuminated area of the cylindricallyconvex surface of the web, and directing said convergent beam onto saidcylindrical surface at an angle of less than thirty degrees therewithand in a plane substantially axial thereto, whereby it illuminates anarea on said surface the width of which area is small relative to boththe radius of curvature of the surface and the length of the areaaxially of the cylindrical surface, and whereby that part of said beamwhich is specularly reflected from said surface emerges therefrom as anapproximately parallel beam, and measuring the intensity of thespecularly reflected beam.

' DONALD B. BRADNER.

GEORGE C. MUNRO,

